Method and apparatus for video coding

ABSTRACT

Aspects of the disclosure provide method and apparatus for video coding. In some examples, an apparatus includes receiving circuitry and processing circuitry. The processing circuitry decodes prediction information of a block from a coded video bitstream. The prediction information is indicative of a reference line selected from a plurality of potential reference lines and an intra prediction mode selected from a set of potential intra prediction modes that is associated with the reference line. Then the processing circuitry reconstructs at least one sample of the block according to the intra prediction mode and at least one reference sample in the reference line.

INCORPORATION BY REFERENCE

This application is a continuation of U.S. Ser. No. 16/200,533 filedNov. 26, 2018, which claims the benefit of priority to U.S. ProvisionalApplication No. 62/651,547, “METHODS AND APPARATUS FOR MULTIPLE LINEINTRA PREDICTION IN VIDEO COMPRESSION” filed on Apr. 2, 2018, the entirecontents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GByte of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reducing aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signal is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used in as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer bitsare required at a given quantization step size to represent the blockafter entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring samples valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Referring to FIG. 1, depicted in the lower right is a subset of ninepredictor directions known from H.265's 35 possible predictordirections. The point where the arrows converge (101) represents thesample being predicted. The arrows represent the direction from whichthe sample is being predicted. For example, arrow (102) indicates thatsample (101) is predicted from a sample or samples to the upper right,at a 45 degree angle from the horizontal. Similarly, arrow (103)indicates that sample (101) is predicted from a sample or samples to thelower right of sample (101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1, on the top right there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in Y dimension (e.g., row index) and its position in Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are referencesamples, that follow a similar numbering scheme. A reference sample islabelled with an R, its Y position (e.g., row index) and X position(column index) relative to block (104). In both H.264 and H.265,prediction samples neighbor the block under reconstruction; therefore nonegative values need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102)—that is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from same reference sample R05. Sample S44 is then predictedfrom reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves can besometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

The mapping of an intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode to codewords, to complex adaptive schemes involving mostprobable modes and similar techniques. In all cases, however, there canbe certain directions that are statistically less likely to occur invideo content than certain other directions. As the goal of videocompression is the reduction of redundancy, those less likely directionswill, in a well working video coding technology, be represented by alarger number of bits than more likely directions.

SUMMARY

Aspects of the disclosure provide method and apparatus for video coding.In some examples, an apparatus includes receiving circuitry andprocessing circuitry. The processing circuitry decodes predictioninformation of a block from a coded video bitstream. The predictioninformation is indicative of a reference line selected from a pluralityof potential reference lines and an intra prediction mode selected froma set of potential intra prediction modes that is associated with thereference line. Then the processing circuitry reconstructs at least onesample of the block according to the intra prediction mode and at leastone reference sample in the reference line.

In some embodiments, the prediction information is indicative of anon-zero reference line and a first set of potential intra predictionmodes associated with the non-zero reference line has a smaller numberof potential intra prediction modes compared to a second set ofpotential intra prediction modes associated with a zero reference line.In an example, the first set of potential intra prediction modescomprises directional intra prediction modes with even mode indexes. Inanother example, the first set of potential intra prediction modes lacksdirectional intra prediction modes with odd mode indexes. In someexamples, the first set of potential intra prediction modes comprises DCand planar modes.

In an embodiment, the first set of potential intra prediction modesincludes only most probable modes. In an example, the first set ofpotential intra prediction modes comprises the most probable modes thatare derived from directional intra prediction modes with even modeindexes.

In some embodiments, the processing circuitry decodes a first signalthat is indicative of the intra prediction mode and selectively decodesa second signal for the reference line based on the intra predictionmode. In an example, the processing circuitry decodes the second signalthat is received after the first signal to determine the reference linewhen the intra prediction mode is a directional intra prediction modewith an even mode index. Then, the processing circuitry determines thereference line to be a default reference line when the intra predictionmode is one of a directional intra prediction mode with an odd modeindex, a planar mode and a DC mode.

In another example, the processing circuitry decodes the second signalthat is received after the first signal to determine the reference linewhen the intra prediction mode is one of most probable modes. Then, theprocessing circuitry determines the reference line to be a defaultreference line when the intra prediction mode is not one of the mostprobable modes.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform the method forvideo coding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a subset of intra prediction modesin accordance with H.265.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 8 shows a schematic 801 that depicts 65 intra predictiondirections.

FIG. 9 shows a diagram 900 for multiple reference line intra directionalprediction according to an embodiment of the disclosure.

FIGS. 10A and 10B show examples of weights for predictions according toan embodiment of the disclosure.

FIG. 11 shows a diagram for deriving illumination compensation (IC)parameters based on neighboring samples.

FIG. 12 shows a diagram illustrating boundary filtering for verticalprediction according to an embodiment of the disclosure.

FIG. 13 shows a diagram of neighboring blocks of a current blockaccording to an example.

FIG. 14 shows a flow chart outlining a process example according to anembodiment of the disclosure.

FIG. 15 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (313), that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Codingor VVC. The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4. The control information for therendering device(s) may be in the form of Supplementary EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (420) may perform entropy decoding/parsing operation on thevideo sequence received from the buffer memory (415), so as to createsymbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values, thatcan be input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (452) has generated to the outputsample information as provided by the scaler/inverse transform unit(451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (453) in the form of symbols (421) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (457) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as document in thevideo compression technology or standard. Specifically, a profile canselect a certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501)(that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . )and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (550) can be configured to have other suitablefunctions that pertain to the video encoder (503) optimized for acertain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4. Brieflyreferring also to FIG. 4, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415), andparser (420) may not be fully implemented in the local decoder (533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies known to a person skilled in the art as, for exampleHuffman coding, variable length coding, arithmetic coding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of Intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A Predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A Bi-directionally Predictive Picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference pictures. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, Supplementary EnhancementInformation (SEI) messages, Visual Usability Information (VUI) parameterset fragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to Intra prediction) makes uses of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first and a second reference picture thatare both prior in decoding order to the current picture in the video(but may be in the past and future, respectively, in display order) areused. A block in the current picture can be coded by a first motionvector that points to a first reference block in the first referencepicture, and a second motion vector that points to a second referenceblock in the second reference picture. The block can be predicted by acombination of the first reference block and the second reference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (603) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621) and anentropy encoder (625) coupled together as shown in FIG. 6.

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform and, in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intra,the general controller (621) controls the switch (626) to select theintra mode result for use by the residue calculator (623), and controlsthe entropy encoder (625) to select the intra prediction information andinclude the intra prediction information in the bitstream; and when themode is the inter mode, the general controller (621) controls the switch(626) to select the inter prediction result for use by the residuecalculator (623), and controls the entropy encoder (625) to select theinter prediction information and include the inter predictioninformation in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata in the frequency domain, and generate the transform coefficients.The transform coefficients are then subject to quantization processingto obtain quantized transform coefficients. In various embodiments, thevideo encoder (603) also includes a residue decoder (628). The residuedecoder (628) is configured to perform inverse-transform, and generatethe decoded residue data. The decoded residue data can be suitably usedby the intra encoder (622) and the inter encoder (630). For example, theinter encoder (630) can generate decoded blocks based on the decodedresidue data and inter prediction information, and the intra encoder(622) can generate decoded blocks based on the decoded residue data andthe intra prediction information. The decoded blocks are suitablyprocessed to generate decoded pictures and the decoded pictures can bebuffered in a memory circuit (not shown) and used as reference picturesin some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard, such asHEVC standard. In an example, the entropy encoder (625) is configured toinclude the general control data, the selected prediction information(e.g., intra prediction information or inter prediction information),the residue information, and other suitable information in thebitstream. Note that, according to the disclosed subject matter, whencoding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive a coded pictures that are part of a coded video sequence, anddecode the coded picture to generate a reconstructed picture. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7.

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example,intra, inter, b-predicted, the latter two in merge submode or anothersubmode), prediction information (such as, for example, intra predictioninformation or inter prediction information) that can identify certainsample or metadata that is used for prediction by the intra decoder(772) or the inter decoder (780) respectively residual information inthe form of, for example, quantized transform coefficients, and thelike. In an example, when the prediction mode is inter or bi-predictedmode, the inter prediction information is provided to the inter decoder(780); and when the prediction type is the intra prediction type, theintra prediction information is provided to the intra decoder (772). Theresidual information can be subject to inverse quantization and isprovided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter QP), and that information may be provided by the entropydecoder (771) (datapath not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503) and (603), and thevideo decoders (310), (410) and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503)and (603), and the video decoders (310), (410) and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503) and (503), and the videodecoders (310), (410) and (710) can be implemented using one or moreprocessors that execute software instructions.

Aspects of the disclosure provide techniques for multiple-line intraprediction.

To capture the arbitrary edge directions presented in natural video, thenumber of directional intra modes can be extended, for example, from 33to 65, and the like. Generally, the planar and DC modes remain the same.The denser directional intra prediction modes apply for all block sizesand for both luma and chroma intra predictions.

FIG. 8 shows a schematic 801 that depicts 65 intra predictiondirections. In some embodiments, a total of 67 intra prediction modesare used. Among the 67 intra prediction modes, intra prediction mode 0is planar mode, intra prediction mode 1 is DC mode, and intra predictionmode 2 to intra prediction mode 66 respectively correspond to the 65intra prediction directions, and are referred to as directional intraprediction modes. As shown in FIG. 8, some of the directional intraprediction modes are identified by dotted arrows, and are associatedwith odd intra prediction mode indexes, and thus are referred to as oddintra prediction modes. The other of directional intra prediction modesare identified by solid arrows, and are associated with even intraprediction mode indexes, and thus are referred to as even intraprediction modes. In this document, the directional intra predictionmodes, as indicated by solid or dotted arrows in FIG. 8 are alsoreferred as angular modes.

In an example, a total of 67 intra prediction modes are used for lumaintra prediction. In some embodiments, to code an intra prediction modeof a current block, a most probable mode (MPM) list of size 6 is builtbased on intra prediction modes of neighboring blocks of the currentblock. For example, six intra prediction modes are selected from theintra prediction modes of the neighboring blocks to form the MPM list.When the intra prediction mode of the current block is not in the MPMlist, a flag is signaled to indicate whether intra prediction modebelongs to the selected intra prediction modes in the MPM list. Inanother example, there are 16 selected intra prediction modes in the MPMlist, the 16 selected intra prediction modes are chosen uniformly asevery fourth angular mode in the angular modes. In another example, 16secondary most probable modes are derived to replace the uniformlyselected intra prediction modes.

According to some aspects of the disclosure, the reference samples usedfor predicting the current block are not restricted to the nearest line(row or column) to the current block. In the method of multiplereference line intra prediction, the index number of candidate referencelines (row or columns) are increased from zero (i.e. the nearest) to N−1for the intra directional modes, where N is an integer equal to orgreater than one. In some examples, the nearest reference line isreferred to as zero reference line, and the other reference lines arereferred to as nonzero reference lines. The reference lines are alsoreferred to as reference tiers in some examples.

FIG. 9 shows a diagram (900) for multiple reference line intradirectional prediction according to an embodiment of the disclosure. Thediagram (900) shows a prediction unit (910) (e.g., 4×4 prediction unit)with multiple reference tiers, such as N reference tiers. Anintra-directional mode could arbitrarily choose one of N reference tiersto generate the predictors. In an example, the predictor p(x,y) isgenerated from one of, for example, the top-left reference samples S0,S1, S2, S3, . . . , SN from different reference tiers. A flag issignaled to indicate which reference tier is chosen for anintra-directional mode. If N is set as 0, the intra directionalprediction method is the same as the traditional method that isrestricted to the nearest line. In FIG. 9, the reference lines arecomposed of six segments together with the top-left reference samples.In this document, a reference tier is also called a reference line. Thecoordinate of the top-left pixel within current block unit is (0,0) andthe top left pixel in the zero reference line is (−1,−1).

In some examples, for the luma component, the neighboring samples usedfor intra prediction sample generations are filtered before thegeneration process. The filtering is controlled by the given intraprediction mode and transform block size. In an example, when the intraprediction mode is DC or the transform block size is equal to 4×4,neighboring samples are not filtered. In another example, when thedistance between the given intra prediction mode and vertical mode (orhorizontal mode) is larger than predefined threshold, the filteringprocess is enabled. In an example, for neighboring sample filtering, [1,2, 1] filter and bi-linear filter can be used.

In some embodiments, position dependent intra prediction combination(PDPC) technique is used in intra prediction. PDPC is an intraprediction technique that invokes a combination of the un-filteredboundary reference samples and HEVC style intra prediction with filteredboundary reference samples. In an example, each prediction samplepred[x][y] located at (x, y) is calculated as follows:

pred[x][y]=(wL×R_(−1,y)+wT≤R_(x,−1)+wTL×R_(−1,−1)+(64−wL−wT−wTL)×pred[x][y]+32)>>6  (Eq. 1)

where R_(x,−1), R_(−1,y) represent the unfiltered reference sampleslocated at top and left of current sample (x, y), respectively,R_(−1,−1) represents the unfiltered reference sample located at thetop-left corner of the current block, and wT, wL, and wTL denoteweights. The weights are calculated by Eq. 2-Eq. 5, width denotes thewidth of the current block, and height denotes the height of the currentblock:

wT=32>>((y<<1)>>shift)  (Eq. 2)

wL=32>>((x<<1)>>shift)  (Eq. 3)

wTL=−(wL>>4)−(wT>>4)  (Eq. 4)

shift=(log 2(width)+log 2(height)+2)>>2  (Eq. 5)

FIG. 10A shows weights for prediction sample (0, 0). In the FIG. 10Aexample, the current block is a 4×4 block, width is 4, height is also 4,thus shift is 1. Then, wT is 32, wL is 32, and wTL is −4.

FIG. 10B shows weights for prediction sample (1,0). In the FIG. 10Bexample, the current block is a 4×4 block, width is 4, height is also 4,thus shift is 1. Then, wT is 32, wL is 16, and wTL is −3.

In some examples, local illumination compensation (LIC) is used. LIC isbased on a linear model for illumination changes. The linear model canbe built based on a scaling factor a and an offset b. The scaling factora and the offset b are referred to as illumination compensation (IC)parameters. LIC can be enabled or disabled adaptively for eachinter-mode coded coding unit (CU).

FIG. 11 shows a diagram for deriving illumination compensation (IC)parameters based on neighboring samples. FIG. 11 shows a current CU(1110) and a reference block (1120). In an example, the reference block(1120) is in a reference picture prior to a current picture having thecurrent CU (1110). The neighboring samples for the current CU (1110) areshown as (1130), and the neighboring samples for the reference block(1120) are shown as (1140). Further in the FIG. 11, a subsamplingtechnique is used to select a subset of neighboring samples. Forexample, when 2:1 subsampling is used, a subset (1150) is selected fromthe neighboring samples (1130) of the current CU (1110), and acorresponding subset (1160) is selected from the neighboring samples(1140) of the reference block (1120). Based on the subset (1150) and thesubset (1160), a least square error method is employed to derive the ICparameters a and b.

In some embodiments, the IC parameters are derived and applied for eachprediction direction separately. When a CU is coded with merge mode, theLIC flag is copied from neighboring blocks, in a way similar to motioninformation copy in merge mode; otherwise, an LIC flag is signaled forthe CU to indicate whether LIC applies or not.

According to a first aspect of the disclosure, for multiple line intraprediction, instead of setting the same number of reference tiers forall blocks, a technique that adaptively selects the number of referencetiers for each block can be used. In this document the index of theclosest reference line is denoted as 0.

In an embodiment, the block sizes of above/left block can be used todetermine the number of reference tiers for the current block. Forexample, when the sizes of above and/or left blocks are larger than M×N,the number of reference tiers for the current block is restricted to L.The M and N can be 4, 8, 16 32, 64, 128, 256 and 512, and L can be 1˜8.In an example, when M and/or N is equal to or larger than 64, L is setto 1. In another example, the ratio of the number of above candidatereference rows to the number of left candidate reference columns is thesame as the ratio of block width to block height. For example, when thecurrent block size is M×N, the number of candidate reference rows abovethe current block is m and the number of candidate reference columnsthat are left to the current block is n, then M:N=m:n.

In another embodiment, the position of last coefficients of left andabove blocks can be used to determine the number of reference tiers forcurrent block. The last coefficients refer to the last non-zerocoefficients in the specified scan order for current block. For example,when the position of last coefficient is within the first M×N region forabove and/or left blocks, the number of reference tiers for currentblock is restricted to L, (e.g. L can be 1˜8), M and N can be 1˜1024. Inan example, when there is no coefficient in above and/or left blocks,the number of reference tiers for current block is restricted to 1. Inanother example, when the coefficients in the above and/or left blocksare within 2×2 top-left region, the number of reference tiers forcurrent block is restricted to 1˜2.

In another embodiment, the pixel values of reference samples in aboveand/or left blocks can be used to determine the number of referencetiers of current block. For example, when the difference betweenreference line with index Li and the reference line with index Lj(Li<Lj) is quite small (e.g., smaller than a threshold), the referenceline Lj will be removed from the reference line list. Li and Lj can be1˜8. In some cases, reference lines with index number larger than 0 areall removed (zero reference line stays), because the difference betweenall the reference lines is quite small. The method to measure thedifference between two reference lines include, but not limited togradient, SATD, SAD, MSE, SNR and PSNR. In an example, when the averageSAD of Li and Lj is less than 2, reference line Lj is removed from thereference line list. In another example, the prediction mode of aboveand/or left mode information can be used to determine the number ofreference tiers for current block. In another example, when theprediction mode of above and/or left blocks is skip mode, the number ofreference tiers for current block is restricted to L. L can be 1˜8.

According to a second aspect of the disclosure, the reference line indexof chroma block can be derived from luma block, both for separated treeor the same tree. Here, the index of the closest reference line isdenoted as 0.

For the same tree, when the reference line index for the co-located lumablock is >=2, the reference line index of current chroma block is setto 1. Otherwise, the reference line index of current chroma block is setto 0.

For the separated tree, when the chroma block just covers one block inluma component, the reference line index derivation algorithm is thesame as the algorithm for the same tree. When the chroma block coversmultiple blocks in luma component, the reference line index derivationalgorithm can be one of the examples. In an example, for the co-locatedblocks in luma component, when the reference line index of majority ofthe blocks are less than 2, the reference line index for current chromablock is derived as 0; otherwise, the reference line index for currentchroma block is derived as 1. The method to measure majority caninclude, but not limited to the region size of the blocks and the numberof the blocks. In another example, for the co-located blocks in lumacomponent, when the reference line index of one block is equal to orlarger than 2, the reference line index for current chroma block isderived as 1; otherwise, the reference line index for current chromablock is derived as 0. In another example, for the co-located blocks inluma component, when the reference line index of majority of the blocksare less than 2, the reference line index for current chroma block isderived as 0; otherwise, the reference line index for current chromablock is derived as 1.

In another embodiment, the number of reference tiers for current chromablock is restricted according to the first aspect of the disclosuredescribed above. After applying the restriction according to the firstaspect of the disclosure, the number of reference tiers is set to LC1.Then, the derivation algorithm that derives the reference line index ofchroma block from luma block is also applied to get the line index forcurrent block LC2. Then, the minimum of LC1 and LC2 is the finalreference line index for current chroma block.

According to a third aspect of the disclosure, different reference linehas different number of intra prediction modes. The index of the closestreference line is denoted as 0.

In an embodiment, when 67 intra prediction modes are used for intraprediction, the zero reference line has 67 modes, the first referenceline has 35 modes, the second reference line has 17 modes, the thirdreference line has 9 modes, and the like.

In another embodiment, nonzero reference lines share the same number ofintra modes, but much less than that of the zero reference line, such asequal to or less than half of the intra prediction modes of the zeroreference line.

In an example, only directional intra prediction modes with even modeindexes are allowed for nonzero reference lines. As illustrated in FIG.8, directional intra prediction modes with odd mode indexes are markedwith dashed arrow while directional intra prediction modes with evenmode indexes are marked with solid arrow.

In another example, only directional intra prediction modes with evenmode index and DC and Planar modes are allowed for nonzero referencelines. In another example, only most probable modes (MPM) are allowedfor nonzero reference lines, including both the first level MPM andsecond level MPM.

In another example, since nonzero reference line is only enabled foreven intra prediction modes, when coding the intra prediction modes, ifa none zero index is signaled, certain intra prediction modes, such asplanar mode, DC mode, and odd intra prediction modes are excluded fromthe MPM derivation and the MPM list, excluded from second level MPMderivation and second level MPM list, and excluded from the remainingnon-MPM mode list.

In another embodiment, the reference line index is signaled aftersignaling of the intra prediction modes, and whether to signal thereference line index is dependent on the signaled intra prediction mode.

In an example, only directional intra prediction modes with even modeindex are allowed for nonzero reference lines. When the signaled intraprediction mode is directional prediction with even mode index, theselected reference line index is signaled; otherwise, only one defaultreference line (zero reference line), e.g., the nearest reference line,is allowed for intra prediction and no index is signaled.

In another example, only most probable modes (MPM) are allowed fornonzero reference lines. When the signaled intra predictions are fromMPMs, the selected reference line index needs to be signaled; otherwise,only one default reference line (zero reference line), e.g., the nearestreference line is allowed for intra prediction and no index is signaled.

In another example, nonzero reference lines are still enabled for alldirectional intra prediction modes, or all intra prediction modes, andthe intra prediction mode index can be used as the context for entropycoding the reference line index.

In another embodiment, for angular intra prediction modes which havederived (not signaled) reference line index, e.g., odd directional intraprediction modes, and/or Planar/DC, multiline reference samples are usedto generate the predictors for current block.

In an example, for angular intra prediction modes which have derived(not signaled) reference line index, the prediction sample value isgenerated using a weighted sum of multiple predictors. Each of themultiple predictors is the prediction generated using one of themultiple reference lines. For example, the weighted sum is using {3, 1}weightings applied on the predictors generated by the first referenceline and second reference line, respectively. In another example, theweightings depend on the block size, block width, block height, sampleposition within the current block to be predicted, and/or intraprediction mode.

In another example, for a given angular prediction mode with odd index,the zero reference line is used to generate one prediction block unitPred₁ and the 1st reference line is used to generate another predictionblock unit Pred₂. Then, the final prediction value for each pixel incurrent block unit is the weighted sum of the two generated predictionblock units. This process can be formulated by the Eq. 6, where W_(i) isthe same value for all the pixels in the same block. Same techniques canbe suitably applied to different blocks regardless of intra predictionmodes and block sizes or dependent on the intra prediction modes andblocks sizes.

Pred′(x,y)=Σ_(i=1) ² W _(i)Pred_(i)(x,y)  (Eq. 6)

In another embodiment, the number of intra prediction modes for eachreference line is derived by the difference between the referencesamples in that reference line. The techniques to measure the differenceinclude, but not limited to gradient, SATD, SAD, MSE, SNR and PSNR.

In an example, when both the above row and left column of the referencesamples are quite similar, the number of modes can be reduced to 4, or9, or 17 or 35 modes. In the example of using 4 modes, the four modesare planar, DC, vertical mode (same column), and horizontal mode (samerow).

In another example, when only above row of the reference samples arequite similar, the modes in vertical-like prediction modes aredown-sampled. In a special case, only mode 50 are kept, and modes35˜mode 49 and mode 51˜mode 66 are excluded. In order to make the totalintra prediction modes as 9 or 17 or 35, the intra prediction modes inhorizontal-like direction is reduced accordingly.

In another example, when only left column of the reference samples arequite similar, the modes in horizontal-like direction are down-sampled.In a special case, only mode 18 are kept, and modes 2˜mode 17 and mode19˜mode 33 are excluded. In order to make the total intra predictionmodes as 9 or 17 or 35, the intra prediction modes in vertical-likedirection is reduced accordingly.

According to a fourth aspect of the disclosure, samples in currentreference line are smoothed based on the neighboring samples in thecurrent reference line and its neighboring reference line(s).

In an embodiment, for each pixel in current reference line, all pixelsin reference lines 1 to L can be used to smooth the pixels in currentline. L is the max allowed reference line number for intra prediction,and L can be 1 to 8.

In an example, for each pixel in the reference line except the boundarypixels, K×L filter is used to smooth each pixel. For the boundarypixels, 1×L filter is used to smooth that pixel. K can be 3, 5, or 7.

The boundary pixels can be filtered or not filtered. When the boundarypixels are filtered, each boundary pixel in the same reference line usesthe same filter. Boundary pixels in the different reference lines canuse different filters. For example, the boundary pixels in the zeroreference line can be filtered by [3,2,2,1] filter, the boundary pixelsin 1st reference line can be filtered by [2,3,2,1] filter, the boundarypixels in 2nd reference line can be filtered by [1,2,3,2] filter, theboundary pixels in 3rd reference line can be filtered by [1,2,2,3]filter.

In another example, for the other pixels, the pixels in each referenceline can use the same filter, and the pixels in different referencelines can use different filters. Alternatively, for the other pixels,the pixels in different position can use different filters. In anexample, the filters are pre-defined, and the encoder does not need tosignal the index of the filter.

In another example, the filtering operation for each reference line canbe intra prediction mode and transform size dependent. The filteringoperation is enabled only when the intra prediction mode and transformsize satisfies certain condition. For example, the filtering operationis disabled when the transform size is equal to 4×4 or smaller.

In another example, rather than rectangular shape, the filter used tosmooth each pixel may have an irregular filter support shape. The filtersupport shape may be pre-defined, and the filter support shape maydepend on any information that is available to both encoder and decoder,including but not limited to: reference line index, intra mode, blockheight and/or width.

In another embodiment, for each pixel in zero reference line, the pixelsin zero reference line and 1st reference line can be used to smooth thatpixel. For each pixel in 1st reference line, the pixels in zeroreference line, 1^(st) reference line, and 2^(nd) reference line can beused to smooth that pixel. For each pixel in 2nd reference line, thepixels in 1st reference line, 2nd reference line, and 3rd reference linecan be used to smooth that pixel. For each pixel in 3rd reference line,the pixels in 2^(nd) reference line and 4^(th) reference line can beused to smooth that pixel. In other words, when four reference lines areused, pixels in the zero reference line and 3rd reference line arefiltered based on the pixels in two reference lines, and for pixels in1st reference line and 2nd reference line, pixels in 3 reference linesare used to filter each pixel.

For example, the filtered pixels in 1st reference line and 2nd referenceline can be computed by one of Eq. 7 to Eq. 10, the filtered pixels inzero reference line can be computed by one of Eq. 11˜Eq. 15, and thefiltered pixels in 3rd reference line can be computed from Eq. 16˜Eq.20. In addition, rounding, such as rounding to zero, rounding topositive infinity or rounding to negative infinity, and the like may beapplied to the filtering calculations.

p′(x,y)=(p(x−1,y)+p(x,y−1)+p(x,y+1)+p(x+1,y)+4×p(x,y))>>3  (Eq. 7)

p′(x,y)=(p(x,y+1)−p(x,y−1)+p(x,y))  (Eq. 8)

p′(x,y)=(p(x−1,y)+p(x−1,y−1)+p(x−1,y+1)+p(x,y−1)+p(x,y+1)+p(x+1,y−1)+p(x+1,y)+p(x+1,y+1)+8×p(x,y))>>4  (Eq.9)

p′(x,y)=(w ₁ ×p(x−1,y)+w ₂ ×p(x−1,y−1)+w ₃ ×p(x−1,y+1)+w ₄ ×p(x,y−1)+w ₅×p(x,y+1)+w ₆ ×p(x+1,y−1)+w ₇ ×p(x+1,y)+w ₈ ×p(x+1,y+1)+w ₈×p(x,y))/(Σ_(i=1) ⁹ w _(i))  (Eq. 10)

p′(x,y)=(p(x−1,y)+p(x,y−1)+p(x+1,y)+5*p(x,y))>>3  (Eq. 11)

p′(x,y)=(p(x−1,y)+p(x,y−1)+p(x+1,y)+p(x,y))>>2  (Eq. 12)

p′(x,y)=(2p(x,y)−p(x,y−1))  (Eq. 13)

p′^((x,y))=(p(x−1,y)+p(x−1,y−1)+p(x,y−1)+p(x+1,y−1)+p(x+1,y)+3*p(x,y))>>3  (Eq.14)

p′(x,y)=(w ₁ *p(x−1,y)+w ₂ *p(x−1,y−1)+w ₃ *p(x,y−1)+w ₄ *p(x+1,y−1)+w ₅*p(x+1,y)+w ₆ *p(x,y))/(Σ_(i=1) ⁶ w _(i))  (Eq. 15)

p′(x,y)=(p(x−1,y)+p(x,y+1)+p(x+1,y)+5*p(x,y))>>3  (Eq. 16)

p′(x,y)=(p(x−1,y)+p(x,y+1)+p(x+1,y)+p(x,y))>>2  (Eq. 17)

p′(x,y)=(2p(x,y)−p(x,y+1))  (Eq. 18)

p′(x,y)=(p(x−1,y)+p(x−1,y+1)+p(x,y+1)+p(x+1,y+1)+p(x+1,y)+3*p(x,y))>>3  (Eq.19)

p′(x,y)=(w ₁ *p(x−1,y)+w ₂ *p(x−1,y=1)+w ₃ *p(x,y+1)+w ₄ *p(x+1,y+1)+w ₅*p(x+1,y)+w ₆ *p(x,y))/(Σ_(i=1) ⁶ w _(i))  (Eq. 20)

According to a fifth aspect of the disclosure, in the current block,samples in different positions may use different combinations ofreference samples with different line indexes to predict.

In an embodiment, for a given intra prediction mode, each reference linei can generate one prediction block Pred_(i). For the pixels atdifferent positions of the current block, different combinations of thegenerated prediction block Pred_(i) can be used to calculate the pixelsat the different positions to generate the final prediction block. To bespecific, for the pixel at position (x,y), Eq. 21 can be used tocalculate the prediction value.

Pred′(x,y)=Σ_(i=0) ^(N) W _(i)Pred_(i)(x,y)  (Eq. 21)

where W_(i) denotes weight for the prediction block Pred_(i) and isposition dependent. In other words, the weighting factors are the samefor the same position, and the weighting factors are different for thedifferent positions.

In another embodiment, given an intra prediction mode, for each sample,a set of reference samples from multiple reference lines are selected,and a weighted sum of these selected set of reference samples iscalculated as the final prediction value. The selection of referencesamples may depend on intra mode and position of prediction sample, andthe weightings may depend on intra mode and position of predictionsample.

In another embodiment, when using Kth reference line (K is positiveinteger) for intra prediction, for each sample, the prediction values bythe zero reference line and the Kth reference line are compared, andwhen prediction value by the Kth reference line is very different fromthe prediction value by the zero reference line (e.g., larger than athreshold), then the prediction value from Kth reference line isexcluded, and the zero reference line may be used instead. The techniqueto measure the difference between prediction value of current positionand that of its neighboring positions include, but not limited togradient, SATD, SAD, MSE, SNR and PSNR. In an example, more than 2prediction values are generated from different reference lines, and themedian (or average, or most frequently appeared) value is used as theprediction of the sample.

In another embodiment, when using Kth reference line for intraprediction, for each sample, the prediction values of the zero referenceline and the Kth reference line are compared, and if line 1 generatevery different prediction value, then the prediction value from line xis excluded, and line 1 may be used instead. The way to measure thedifference between prediction value of current position and that of itsneighboring positions include, but not limited to gradient, SATD, SAD,MSE, SNR and PSNR.

According to a sixth aspect of the disclosure, after intra prediction,instead of only using the pixels in the nearest reference line, thepixels in multiple reference lines are used to filter the predictionvalue of each block. For example, PDPC is extended for multiple lineintra prediction. Each prediction sample pred[x][y] located at (x, y) iscalculated as Eq. 22:

pred[x][y]=(Σ_(i=1) ⁻¹ *R _(i,y)+Σ_(i=m) ⁻¹ wT _(i) *R _(x,i)+Σ_(i=m) ⁻¹wTL _(i) *TL _(i,i)+(64−Σ_(i=m) ⁻¹ wL _(i)−Σ_(i=m) ⁻¹ wT _(i)−Σ_(i=m) ⁻¹wTL _(i))*pred[x][y]+32)>>6  (Eq. 22)

where m can be −8 to −2.

In an example, reference samples in the nearest 2 lines are used tofilter the samples in current block. For top-left pixel, only thetop-left sample in the first row are used, such as shown in Eq. 23:

pred[x][y]=Σ_(i=2) ⁻¹ wL _(i) *R _(i,y)+Σ_(i=−2) ⁻¹ wT _(i) *R _(x,i)+wTL ⁻¹ *TL _(−1,−1)+(64−Σ_(i=−2) ⁻¹ wL _(i)−Σ_(i=−2) ⁻¹ wT _(i) −wTL⁻¹)*pred[x][y]+32)>>6  Eq. 23

In another example, boundary filters can be extended to multiplereference lines. For example, after DC prediction, the pixels in thefirst several columns and the first several rows are filtered by theneighboring reference pixels. The pixels in the first column can befiltered using Eq. 24, the pixels in the first row can be filter suingEq. 25,

p′(x,y)=(Σ_(i=m) ⁻¹ wL _(i) *R _(i,y)+(64−Σ_(i=m) ⁻¹ wL_(i))*p(x,y))>>6  (Eq. 24)

p′(x,y)=(Σ_(i=m) ⁻¹ wT _(i) *R _(x,i)+(64−Σ_(i=m) ⁻¹ wT_(i))*p(x,y))>>6  (Eq. 25)

In some special case, the pixels in the first column can be filteredusing Eq. 26, the pixels in the first row can be filter using Eq. 27:

p′(0,y)=p(0,y)+R _(−1,y) −R _(−2,y)  (Eq. 26)

p′(x,0)=p(x,0)+R _(x,−1) −R _(x,−2)  (Eq. 27)

In another example, after vertical prediction, the pixels in the firstseveral columns can be filtered using Eq. 28; and after horizontalprediction, the pixels in the first several rows can be filtered usingEq. 29:

p′(x,y)=Σ_(i=m) ⁻¹ wL _(i)*(R _(i,y) −R _(i,i))+p(x,y)  (Eq. 28)

p′(x,y)=Σ_(i=m) ⁻¹ wT _(i)*(R _(x,i) −R _(i,i))+p(x,y)  (Eq. 29)

In another example, for vertical/horizontal prediction, when a nonzeroreference line is used to generate the prediction sample, the zeroreference line and the corresponding pixel in the nonzero reference lineis used for boundary filtering.

FIG. 12 shows a diagram illustrating boundary filtering for verticalprediction according to an embodiment of the disclosure. In FIG. 12,1^(st) reference line is used to generate the prediction sample forcurrent block and the pixels in vertical direction (shown as 1220) areused for vertical prediction. After the vertical prediction, the pixelswith diagonal texture in the zero reference line (shown as 1230) and thepixel with diagonal texture in the 1^(st) reference line (shown as 1240)are used to filter the first several columns in current block. Thefiltering process after the vertical prediction can be formulated by Eq.30, where m denotes the selected reference line index, and m can be 2˜8.n is the number of right shift bits, and can be 1˜8. For horizontalprediction, the filtering process can be formulated by Eq. 31.

p′(x,y)=p(x,y)+(p(−1,y)−p(−1,−m))>>n  (Eq. 30)

p′(x,y)=p(x,y)+(p(x,−1)−p(−m,−1))>>n  (Eq. 31)

In another embodiment, when a nonzero reference line is used, afterdiagonal predictions, such as mode 2 and mode 66 in FIG. 8, pixels alongthe diagonal direction from the zero reference line to the nonzeroreference line are used for filtering the pixels in the first severalcolumns/rows of current block. To be specific, after mode 2 prediction,the pixels in the first several rows can be filtered using Eq. 32. Aftermode 66 prediction, the pixels in the first several columns can befiltered using Eq. 33. m denotes the nonzero reference line index forcurrent block, and it can be 2˜8. n is the number of right shift bits,it can be 2˜8. W_(i) is the weighting coefficients, and is an integer.

p′ ^((x,y))=(Σ_(i=1) ^(m) W _(i) R(x+i,−i)+(2^(n)−Σ_(i×1) ^(m) W_(i))*p(x,y)+2^(n-1))>>n  (Eq. 32)

p′ ^((x,y))=(Σ_(i=1) ^(m) W _(i) R(−i,y+i)+(2^(n)−Σ_(i=1) ^(,) W _(i)*p(x,y)+2^(n-1))>>n  (Eq. 33)

According to a seventh aspect of the disclosure, for multiple referenceline intra prediction, modified DC and planar modes are added fornonzero reference line. In an embodiment, for the planar mode, when adifferent reference line is used, different pre-defined top-right andbottom-left reference samples are used to generate the predictionsamples. In another embodiment, when a different reference line is used,different intra smoothing filter is used.

In an embodiment, for DC mode, for the zero reference line, all thepixels in the above row and the left column are used to calculate the DCvalue. For the nonzero reference line, only some of the pixels are usedto calculate the DC value. For example, above pixels in the zeroreference line are used to calculate the DC values for 1st referenceline, left pixels in the zero reference line are used to calculate theDC values for 2nd reference line, half of left pixels and half of theabove pixels in zero reference line are used to calculate the DC valuesfor the 3rd reference line. In another embodiment, for DC mode, allreference pixels in all available candidate lines (rows and columns) areused to calculate the DC predictor.

According to an eighth aspect of the disclosure, techniques for multiplereference line can be extended to the IC mode. In an embodiment, the ICparameters are calculated using multiple above/left reference lines. Inanother embodiment, the reference line that is used to calculate ICparameters can be signaled.

According to a ninth aspect of the disclosure, the index of thereference line is signaled. In an embodiment, the reference line indexis signaled using variable length coding. The closer to the currentblock in distance, the shorter the codeword. For example, if thereference line index is 0, 1, 2, 3, with 0 being the closest to thecurrent block and 3 the furthest, the codewords for them are 1, 01, 001,000, where 0 and 1 can be alternated.

In another embodiment, the reference line index is signaled using fixedlength coding. For example, if the reference line index is 0, 1, 2, 3,with 0 being the closest to the current block and 3 the furthest, thecodewords for them are 10, 01, 11, 00, where 0 and 1 can be alternatedand the order may be altered.

In another embodiment, the reference line index is signaled usingvariable length coding, where the order of the indices in the codewordtable (from the shortest codeword to the longest) is as follows: 0, 2,4, . . . 2k, 1, 3, 5, . . . 2k+1 (or 2k−1). Index 0 indicates thereference line which is the closest to the current block and 2k+1 thefurthest.

In yet another embodiment, the reference line index is signaled usingvariable length coding, where the order of the indices in the codewordtable (from the shortest codeword to the longest) is as follows: theclosest, the furthest, 2^(nd) closest, 2^(nd) furthest, . . . and so on.In one specific example, if the reference line index is 0, 1, 2, 3, with0 being the closest to the current block and 3 the furthest, thecodewords for them are 0 for index 0, 10 for index 3, 110 for index 2,111 for index 1. The codewords for reference line index 1 and 2 may beswitched. The 0 and 1 in codewords may be altered.

According to a tenth aspect of the disclosure, when the number of abovereference lines (rows) is different from the number of left referencelines (columns), the index of the reference line is signaled. In anembodiment, if the number of above reference lines (rows) is M and thenumber of left reference lines (columns) is N, then the reference lineindices for max (M, N) may use any of the signaling techniques describedabove, or their combinations. The reference line indices for min(M, N)take a subset of the codewords from the codewords used for indicatingreference line indices for max(M, N), usually the shorter ones. Forexample, if M=4, N=2, and the codewords used to signal M (4) referenceline indices {0, 1, 2, 3} are 1, 01, 001, 000, then the codewords usedto signal N (2) reference line indices {0, 1} are 1, 01.

In another embodiment, if the number of above reference lines (rows) isM and the number of left reference lines (columns) is N, and if M and Nare different, then the reference line indices for signaling abovereference line (row) index and left reference line (column) index may beseparate and independently use any signaling technique described aboveor their combinations.

According to an eleventh aspect of the disclosure, the maximum number ofreference lines that may be used for intra prediction may be constrainedto be no more than the number of reference lines used in other codingtools, such as deblocking filter or template matching based intraprediction, in order to potentially save the pixel line buffer.

According to a twelfth aspect of the disclosure, the multiple line intraprediction and other coding tools/modes may interact. In an embodiment,the usage and/or signaling of other syntax elements/coding tools/modes,including but not limited to: cbf, last position, transform skip,transform type, secondary transform index, primary transform index, PDPCindex, may depend on the multi-line reference line index. In oneexample, when multi-line reference index is nonzero, transform skip isnot used, and transform skip flag is not signaled. In another example,the context used for signaling other coding tools, e.g., transform skip,cbf, primary transform index, secondary transform index, may depend onthe value of multi-line reference index.

In another embodiment, the multi-line reference index may be signaledafter other syntax elements, including but not limited to: cbf, lastposition, transform skip, transform type, secondary transform index,primary transform index, PDPC index, and the usage and/or signaling ofmulti-line reference index may depend on other syntax elements.

According to a thirteenth aspect of the disclosure, the reference lineindex can be used as the context for entropy coding another syntaxelement, including, but not limited to intra prediction mode, MPM index,primary transform index, secondary transform index, transform skip flag,coding block flag (CBF) and transform coefficients, or vice versa.

According to a fourteenth aspect of the disclosure, the reference lineinformation can be included into the MPM list. When the prediction modeof current block is the same as one candidate in MPM list, both of theintra prediction and the selected reference line of the selectedcandidate are applied for current block, and the intra prediction modeand reference line index are not signaled. In addition, the number ofthe MPM candidates for different reference line indexes are predefined.

In one embodiment, the number of MPMs for each reference line index ispredefined and can be signaled as a higher level syntax element, such asin sequence parameter set (SPS), picture parameter set (PPS), sliceheader, Tile header, coding tree unit (CTU) header, or as a commonsyntax element or parameter for a region of a picture. As a result, thelength of MPM list can be different in different sequences, pictures,slices, Tiles, group of coding blocks or a region of a picture. Forexample, the number of MPMs for the zero reference line is 6, and thenumber of MPMs with each of other reference line indices is 2. As aresult, if the total reference line number is 4, the total number of MPMlist is 12.

In another embodiment, all intra prediction modes together with theirreference line index in the above, left, top-left, to-right, andbottom-left block are included into the MPM list.

FIG. 13 shows a diagram of neighboring blocks of a current blockaccording to an example. As illustrated in FIG. 13, A is bottom-leftblock, B, C, D, and E are left blocks, F is top-left block, G and H aretop blocks, and I is top-right block. After adding modes of theneighboring blocks into MPM list, if the number of MPM candidate withgiven reference line number is less than the predefined number, defaultmodes are used to fill the MPM list.

In one example, for MPM candidate with the zero reference line, Planar,DC, Vertical, Horizontal, Mode 2 (diagonal mode), and Mode 66 (diagonalmode) are added into MPM list in this order until the length of MPMcandidate with the zero reference line reaches the predefined number.For MPM candidate with other reference line index, Vertical andHorizontal Modes are added into MPM list in this order.

In another embodiment, when the mode of current block is equal to one ofthe candidate in MPM list, the reference line index is not signaled. Ifthe mode of current block is not equal to any candidate in MPM list,reference line index is signaled. In one example, when the zeroreference line is used for current block, the second level MPM mode isstill used, but the second level MPM only includes the intra predictionmode information. In another example, for nonzero reference lines, thesecond level MPM is not used, and fixed length coding is used to codethe remaining mode.

FIG. 14 shows a flow chart outlining a process (1400) according to anembodiment of the disclosure. The process (1400) can be used in thereconstruction of a block coded in intra mode, so to generate aprediction block for the block under reconstruction. In variousembodiments, the process (1400) are executed by processing circuitry,such as the processing circuitry in the terminal devices (210), (220),(230) and (240), the processing circuitry that performs functions of thevideo encoder (303), the processing circuitry that performs functions ofthe video decoder (310), the processing circuitry that performsfunctions of the video decoder (410), the processing circuitry thatperforms functions of the intra prediction module (452), the processingcircuitry that performs functions of the video encoder (503), theprocessing circuitry that performs functions of the predictor (535), theprocessing circuitry that performs functions of the intra encoder (622),the processing circuitry that performs functions of the intra decoder(772), and the like. In some embodiments, the process (1400) isimplemented in software instructions, thus when the processing circuitryexecutes the software instructions, the processing circuitry performsthe process (1400). The process starts at (S1401) and proceeds to(S1410).

At (S1410), prediction information of a block is decoded from a codedvideo bitstream. The prediction information is indicative of an intraprediction mode and a reference line. The reference line is selectedfrom a plurality of potential reference lines. The intra prediction modeis selected from a set of potential intra prediction modes that isassociated with the reference line. Different reference lines havedifferent sets of potential intra prediction modes that are respectivelyassociated with the different reference lines.

At (S1420), determine the intra prediction mode and the reference line.

At (S1430), samples of the block are constructed according to the intraprediction mode and reference samples in the reference line. Then theprocess proceeds to (S1499) and terminates.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 15 shows a computersystem (1500) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 15 for computer system (1500) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1500).

Computer system (1500) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1501), mouse (1502), trackpad (1503), touchscreen (1510), data-glove (not shown), joystick (1505), microphone(1506), scanner (1507), camera (1508).

Computer system (1500) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1510), data-glove (not shown), or joystick (1505), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1509), headphones(not depicted)), visual output devices (such as screens (1510) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1500) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1520) with CD/DVD or the like media (1521), thumb-drive (1522),removable hard drive or solid state drive (1523), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1500) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1549) (such as, for example USB ports of thecomputer system (1500)); others are commonly integrated into the core ofthe computer system (1500) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1500) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1540) of thecomputer system (1500).

The core (1540) can include one or more Central Processing Units (CPU)(1541), Graphics Processing Units (GPU) (1542), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1543), hardware accelerators for certain tasks (1544), and so forth.These devices, along with Read-only memory (ROM) (1545), Random-accessmemory (1546), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1547), may be connectedthrough a system bus (1548). In some computer systems, the system bus(1548) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1548),or through a peripheral bus (1549). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1541), GPUs (1542), FPGAs (1543), and accelerators (1544) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1545) or RAM (1546). Transitional data can be also be stored in RAM(1546), whereas permanent data can be stored for example, in theinternal mass storage (1547). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1541), GPU (1542), massstorage (1547), ROM (1545), RAM (1546), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1500), and specifically the core (1540) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1540) that are of non-transitorynature, such as core-internal mass storage (1547) or ROM (1545). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1540). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1540) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1546) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1544)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

APPENDIX A: ACRONYMS

JEM: joint exploration modelVVC: versatile video codingBMS: benchmark set

MV: Motion Vector HEVC: High Efficiency Video Coding SEI: SupplementaryEnhancement Information VUI: Video Usability Information GOPs: Groups ofPictures TUs: Transform Units, PUs: Prediction Units CTUs: Coding TreeUnits CTBs: Coding Tree Blocks PBs: Prediction Blocks HRD: HypotheticalReference Decoder SNR: Signal Noise Ratio CPUs: Central Processing UnitsGPUs: Graphics Processing Units CRT: Cathode Ray Tube LCD:Liquid-Crystal Display OLED: Organic Light-Emitting Diode CD: CompactDisc DVD: Digital Video Disc ROM: Read-Only Memory RAM: Random AccessMemory ASIC: Application-Specific Integrated Circuit PLD: ProgrammableLogic Device LAN: Local Area Network

GSM: Global System for Mobile communications

LTE: Long-Term Evolution CANBus: Controller Area Network Bus USB:Universal Serial Bus PCI: Peripheral Component Interconnect FPGA: FieldProgrammable Gate Areas

SSD: solid-state drive

IC: Integrated Circuit CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof

What is claimed is:
 1. A method for video decoding, comprising: decodingprediction information of a block from a coded video bitstream, theprediction information being indicative of a reference line selectedfrom a plurality of potential reference lines and an intra predictionmode selected from a set of potential intra prediction modes that isassociated with the reference line; and reconstructing at least onesample of the block according to the intra prediction mode and at leastone reference sample in the reference line, wherein the predictioninformation includes index information of the reference line only whenthe intra prediction mode is one of a subset of potential intraprediction modes associated with an adjacent reference line of theblock.
 2. The method of claim 1, wherein each of the subset of potentialintra prediction modes is further associated with at least onenon-adjacent reference line of the block.
 3. The method of claim 1,wherein the subset of potential intra prediction modes comprisesdirectional intra prediction modes with even mode indexes.
 4. The methodof claim 1, wherein the subset of potential intra prediction modes lacksdirectional intra prediction modes with odd mode indexes.
 5. The methodof claim 3, wherein the subset of potential intra prediction modescomprises DC and planar modes.
 6. The method of claim 3, wherein thesubset of potential intra prediction modes comprises DC mode.
 7. Themethod of claim 1, wherein the subset of potential intra predictionmodes comprises only most probable modes.
 8. The method of claim 7,wherein the subset of potential intra prediction modes comprises themost probable modes that are derived from directional intra predictionmodes with even mode indexes.
 9. The method of claim 1, wherein decodingthe prediction information of the block from the coded video bitstreamfurther comprises: decoding a first signal that is indicative of theintra prediction mode; and selectively decoding a second signal forindex information of the reference line based on the intra predictionmode.
 10. The method of claim 9, further comprising: decoding the secondsignal that is received after the first signal to determine thereference line when the intra prediction mode is a directional intraprediction mode with an even mode index; and determining the referenceline to be a default reference line when the intra prediction mode isone of a directional intra prediction mode with an odd mode index, aplanar mode and a DC mode.
 11. The method of claim 9, furthercomprising: decoding the second signal that is received after the firstsignal to determine the reference line when the intra prediction mode isone of most probable modes; and determining the reference line to be adefault reference line when the intra prediction mode is not one of themost probable modes.
 12. An apparatus for video decoding, comprising:processing circuitry configured to: decode prediction information of ablock from a coded video bitstream, the prediction information beingindicative of a reference line selected from a plurality of potentialreference lines and an intra prediction mode selected from a set ofpotential intra prediction modes that is associated with the referenceline; and reconstruct at least one sample of the block according to theintra prediction mode and at least one reference sample in the referenceline, wherein the prediction information includes index information ofthe reference line only when the intra prediction mode is one of asubset of potential intra prediction modes associated with an adjacentreference line of the block.
 13. The apparatus of claim 12 wherein eachof the subset of potential intra prediction modes is further associatedwith at least one non-adjacent reference line of the block.
 14. Theapparatus of claim 12, wherein the subset of potential intra predictionmodes comprises directional intra prediction modes with even modeindexes.
 15. The apparatus of claim 12, wherein the subset of potentialintra prediction modes lacks directional intra prediction modes with oddmode indexes.
 16. The apparatus of claim 14, wherein the subset ofpotential intra prediction modes comprises DC and planar modes.
 17. Theapparatus of claim 14, wherein the subset of potential intra predictionmodes comprises DC mode.
 18. The apparatus of claim 12, wherein thesubset of potential intra prediction modes comprises only most probablemodes.
 19. The apparatus of claim 18, wherein the subset of potentialintra prediction modes comprises the most probable modes that arederived from directional intra prediction modes with even mode indexes.20. The apparatus of claim 12, wherein the decoding the predictioninformation of the block from the coded video bitstream furthercomprises: decoding a first signal that is indicative of the intraprediction mode; and selectively decoding a second signal for the indexinformation of the reference line based on the intra prediction mode.21. The apparatus of claim 20, further comprising: decoding the secondsignal that is received after the first signal to determine thereference line when the intra prediction mode is a directional intraprediction mode with an even mode index; and determining the referenceline to be a default reference line when the intra prediction mode isone of a directional intra prediction mode with an odd mode index, aplanar mode and a DC mode.
 22. A non-transitory computer-readable mediumstoring instructions which when executed by a computer for videodecoding cause the computer to perform: decoding prediction informationof a block from a coded video bitstream, the prediction informationbeing indicative of a reference line selected from a plurality ofpotential reference lines and an intra prediction mode selected from aset of potential intra prediction modes that is associated with thereference line; and reconstructing at least one sample of the blockaccording to the intra prediction mode and at least one reference samplein the reference line, wherein the prediction information includes indexinformation of the reference line only when the intra prediction mode isone of a subset of potential intra prediction modes associated with anadjacent reference line of the block.